Quick Answer
A toilet that keeps running almost always comes down to one of three parts inside the cistern: a worn flapper that no longer seals, a faulty fill valve that never shuts off, or a float set too high so water spills into the overflow tube. All three are cheap to fix — but left alone they can waste over 4,000 litres a month.
That faint hiss of water in the bathroom at night. The cistern that refills itself every few minutes even though nobody has touched it. A running toilet is one of the most common calls we get — and one of the easiest to ignore, right up until the water bill arrives.
The good news: the cause is nearly always one of three worn parts, and two of them are simple to check yourself.
How a toilet actually works
Lift the cistern lid and you’ll see two things. At the bottom sits the flapper (or flush valve) — a rubber seal that lifts when you flush and drops back to hold water in the tank. On one side is the fill valve, controlled by a float that rises with the water level and shuts the valve off when the tank is full.
When a toilet runs continuously, it means water is escaping the tank faster than it should — so the fill valve keeps topping it up. Find where the water is leaving and you’ve found your problem.
1. A worn flapper (most common)
Rubber flappers harden and warp with age and years of chlorinated water. Once the seal is imperfect, water trickles from the tank into the bowl continuously, and the fill valve keeps running to compensate.
The classic test: add a few drops of food colouring to the cistern and wait 15 minutes without flushing. If colour appears in the bowl, your flapper is leaking. A replacement is a few dollars and takes ten minutes with no tools.
2. A faulty fill valve
If the flapper is sealing but the toilet still hisses, the fill valve itself may be failing to shut off completely. You’ll often hear a constant high-pitched trickle even when the tank looks full.
Fill valves wear out, jam with sediment, or lose their seal. Replacing one is a slightly bigger job than a flapper and involves turning off the water supply and draining the tank — a common point where DIY tips over into a plumber’s call.
3. The float is set too high
If water is draining into the plastic overflow tube in the middle of the cistern, the float is set too high — the tank fills past the safe line and spills continuously into the tube.
Look for the water level sitting right at the top of the overflow tube. Lowering the float (bending the arm on older toilets, or adjusting the clip/screw on modern ones) so the water stops about 20mm below the tube usually fixes it in seconds.
What a running toilet is really costing you
A steadily running toilet can waste 100 to 200 litres a day — quietly, around the clock. Over a month that’s 3,000 to 6,000 litres, and it lands straight on your Sydney Water bill. We regularly meet homeowners chasing a “mystery” bill spike that turns out to be one running toilet in a rarely-used bathroom.
If you want to check whether a hidden toilet leak is inflating your usage, our guide on how to read your Sydney water bill walks through spotting it on the meter.
5-minute DIY checks before you call
Before anything else, run through these:
1. The dye test. Food colouring in the tank, wait 15 minutes, check the bowl. Colour in the bowl = flapper leak.
2. Check the water level. If it’s at or above the overflow tube, adjust the float down.
3. Wiggle the flush handle. If the running stops when you jiggle it, the flapper or chain is catching and not sealing.
If none of these solve it, or the problem keeps coming back, the fill valve or cistern seals likely need replacing — and that’s where we come in.
FAQs
Why does my toilet run for a few seconds every so often on its own?
That’s called “phantom flushing” and it’s a textbook sign of a slowly leaking flapper. Water escapes the tank so gradually that the fill valve kicks in briefly every few minutes to top it up. Replace the flapper and it stops.
Is it worth repairing an old toilet or should I replace it?
If the cistern and bowl are sound, replacing worn internal parts is far cheaper than a new toilet. But if the toilet is very old, cracked, or a water-guzzling single-flush model, upgrading to a modern dual-flush unit pays for itself in water savings over a few years.
Do you charge a call-out fee to fix a running toilet?
No — Mr. Clog charges $0 call-out and gives you an upfront fixed price before any work starts, so you know the cost before we begin.
Toilet Still Running After All That?
Mr. Clog repairs running toilets, cisterns, and taps across Sydney — fast, tidy, and fixed-price with no call-out fee.